Tennessee, US

State Fossil:

Pterotrigonia thoracicaThis small bivalve lived in the soft, clay sands of the shallow sea that encroached onto North America during the Cretaceous Period (~ 100-65 million years ago). Frequently these fossils are found in large concentrations, indicating mass die-offs, the causes of which remain unknown.

Paleontology and geology

The Precambrian: In the Precambrian, the future state of Tennessee lay beneath marine waters far south of the equator. Sediments that accumulated on the sea floor were later metamorphosed and intruded by molten material during mountain building. These igneous and metamorphic rocks are now exposed in the Blue Ridge Mountains along the eastern border of Tennessee.
The Paleozoic: During this time, Tennessee lay along the southern margin of future North America as the continent drifted north toward the equator. Shallow sea water covered the state through most of this interval (Cambrian through Early Carboniferous), and the sea floor was home to a variety of animals, including brachiopods, trilobites, crinoids, bryozoans, and corals. In the Late Carboniferous (Pennsylvanian), mountain-building to the east produced vast amounts of sediment that was carried by westward-flowing rivers into the shallow sea. Huge, swampy deltas developed. These low-lying areas were lush with scale trees, horsetail rushes, and other plants that would eventually produce Tennessee’s coal deposits. The state lay above sea level by the end of the era, and erosion outpaced deposition.
The Mesozoic: Tennessee lay above sea level for much of the Mesozoic, and erosion outpaced deposition. The sea advanced across the western part of the state in the Cretaceous, bringing a return to marine conditions in that region. Crinoids, clams, oysters, and snails thrived in the shallow waters, while dinosaurs walked the dry land farther east.
The Cenozoic: During the Early Cenozoic (Tertiary), warm, tropical marine waters periodically advanced across western Tennessee, while the rest of the state remained above sea level. Molluscs and other typical marine organisms have left their fossils in the marine rocks; elephants, tapirs, alligators, and other animals roamed a landscape of swamps, forests, and rivers. The ice sheets that covered parts of North America in the Late Cenozoic (Quaternary) did not extend as far south as Tennessee. However, fossils of mastodons found in the state tell us that the climate did become significantly cooler during this time.